Remote laser assisted biological aerosol standoff detection in atmosphere

ABSTRACT

A method used to detect and identify biological substances suspended in air in the form of aerosols or clouds including generating a remote infrared light beam directed toward the atmospheric contamination, producing an ultraviolet light beam from the infrared light beam by compression via the air through which the IR beam travels, and producing fluorescence of the atmospheric contamination, when the generated ultraviolet light contacts the atmospheric contamination. The fluorescent signals are then processed in order to identify the nature of the atmospheric contamination.

BACKGROUND OF THE INVENTION

A common method used to detect and identify biological substancessuspended in air in the form of aerosols or clouds involves air samplecollection in the field and their subsequent analysis in mobilelaboratories. While this approach can be acceptably accurate, it hasmany disadvantages. Among these disadvantages includes being dangerousto personnel conducting the tests as they are exposed to hazardousbiological agents. In addition, transporting the test equipment to thetesting site can be difficult, especially if the test site is remoteand/or in harsh terrain. Furthermore, the testing can be time-consumingin order to test large areas, thus decreasing the value of the testingby delaying obtaining the test results.

An alternative method for the remote sensing of biological substances,for example, would be a standoff detection such as a LIDAR (lightdetection and ranging) using an UV laser source. LIDAR technologyemploys laser pulses to determine the distance to an object or surface,for example. Backscattered fluorescence signals from the laser pulsesencountering objects or materials indicate the presence and the locationof any potential microscopic biological materials. The characteristicspectral information may also enable identification of these potentialmicroscopic biological materials. However, there are still problemsassociated with this method. Employing a LIDAR system causes manymolecules of interest to be directly excited by radiation in the vacuumultraviolet (VUV) region, which, unfortunately, is heavily absorbed bythe Earth's atmosphere for wavelengths below 300 mm. Thus, the LIDARsystem limits the UV LIDAR detection range to only a few hundred meters,especially in high ozone urban environment.

BRIEF SUMMARY OF THE INVENTION

To overcome the limited UV LIDAR detection range, a locally generated UVradiation excitation source is preferred. This source is preferablyplaced at a remote location, instead of launching an intense UV laserfrom a distance. Generating the UV radiation at a remote location,directed towards the atmospheric contaminant through nonlinear processesassociated with propagation of intense laser pulses, is preferable.

Accordingly, one object of an embodiment of the present invention is theability to test for contamination from a remote location from thecontaminant site.

Another object of an embodiment of the present invention is the abilityto obtain real-time test results for an area of contamination.

These and other objects are achieved by an embodiment of the presentinvention including a method for detecting atmospheric contaminationcomprising generating an infrared light beam remotely from anatmospheric contamination region, directing an infrared light beamtoward the atmospheric contamination region, wherein linearlycompressing a longitudinal component of said infrared light beam andincreasing intensity of said infrared light beam wherein non-linearlycompressing a transverse component of said infrared light beam, whereinconverting said linearly compressed infrared light beam to anultraviolet light source, and wherein producing fluorescence of theatmospheric contamination, when said generated ultraviolet light sourcecontacts the atmospheric contamination at the atmospheric contaminationregion.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages, and novel features will be morereadily appreciated from the following detailed description when read inconjunction with the accompanying drawings, in which:

FIG. 1 is illustrative of a block diagram of an embodiment of thepresent invention;

FIG. 2 is a graph illustrating the UV portion of the self-phasemodulated spectrum of the laser beam containing filaments;

FIG. 3 is a graph illustrating the spectrum of a third harmonic of thefundamental frequency;

FIG. 4 is a graph illustrating a normalized signal from photo multipliertube of light scattered by silica powder (solid line) and a normalizedsignal of the scattered and fluorescent light (dashed line) when the nonfluorescent aerosol is replaced by a biological aerosol containingtryptophan.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention includes a remote detector systemfor biological aerosols that employs broad bandwidth laser pulses atatmospheric transmitting wavelengths such as infrared (IR). The detectorsystem delivers laser pulses from a remote location to the contaminatedsite via locally generated UV radiation, through the process of lightfilament formation and atmospheric breakdown. The generated UV radiationis used as the light source to excite fluorescence in the contaminatedarea containing contaminants such as biological substances. Thefluorescent response is processed, via a processor, to map out thecontaminated area and identify the contaminating agent.

FIG. 1 depicts a system 10 for remote detection of air contaminants, forexample biological aerosols. The detection system employs a laser 15which emits a laser pulse 20, typically in the infrared (IR) range fromabout 700 nm.-1500 nm. This initial pulse 20 is preferably frequencychirped, with the wavelength and the frequency being a function of time.The chirped pulse can be generated by optical grating-based dispersionsuch as that occurring in a chirped pulse amplifier laser, or by anysuitable method. The laser pulse 20 comes into contact with the airwhich acts to compress the laser pulse 20 into a compressed laser pulse25, typically in the picosecond range. More specifically, the laserpulse 20 is linearly compressed in a longitudinal direction thusincreasing the power of the laser pulse 20. In addition, with thisincreased power, the laser pulse is non-linearly focused in a transversedirection resulting in a higher laser intensity or, in other words,higher power/unit area. The present invention employs a compressed laserpulse 25 which implicitly refers to a longitudinally compressed andtransversely focused laser pulse. The laser is able to achieve a higherlaser intensity with the laser source being remote, approximately up to10 Km. from the contamination site. Accordingly, this higher laserintensity is able to generate UV light via three processes: 1. aself-phase modulated laser pulse spectrum 2. a third harmonic of thelaser pulse (at one-third the wavelength of the IR laser. i.e., 267 nmfor an IR wavelength of 800 nm.) 3. plasma radiation spectrum from therecombination of electrons and partially ionized air to produce a plasmathat exists for a short duration

A feature of an embodiment of the present invention includes employing alaser pulse in the IR spectrum, because upon compression the laser pulseresults in a UV spectrum laser (less than 300 nm.).

Once the linear and non-linear components of the compressed laser pulsecome into contact with the atmospheric contaminants 30, the contaminantsfluoresce in the area of contamination 35. The return fluoresced signalis then sent to processors, via sensors (not shown), for example, so theinformation collected about the offending contaminants at the remotesite is compared to known signatures of biological contaminants todetermine the identity of the offending contaminant.

The non-linear transverse focusing results in laser filaments asdiscussed below.

The compressed laser pulse comprises an ultraviolet laser pulse furthercomprising three components including a self-phase modulated laser pulsespectrum, a third harmonic of the laser pulse (at one-third thewavelength of the IR laser, i.e., 267 nm for an IR wavelength of 800nm.), and a plasma radiation spectrum from the recombination ofelectrons and partially ionized air to produce a plasma that exists fora short duration. This plasma is readily recombined. This short durationplasma is produced when electrons are stripped from atoms to form ions,the electrons are then recombined with the available ions within theionized air in order to produce the above referenced ultraviolet signal.

Among the three components mentioned above, the third harmonic of thelaser pulse is a preferable marker for examining potential contaminants.Specifically, the third harmonic offers benefits including the fact thata significant number of biological markers have been shown to fluoresceunder the third harmonic, such as tryptophan. In addition, the thirdharmonic is a more dominating mechanism for generating UV spectrumbecause the third harmonic can be a coherent generation through phasematching. The third harmonic is also more efficient in exciting thebiological markers due to the fact that the third harmonic has higherspectral intensity with due to the narrower spectral bandwidthassociated with the third harmonic.

In order to achieve appropriate control of the laser pulse formation andair breakdown, an embodiment of the present invention allows adjustmentof the initial laser pulse parameters such as laser placement withrespect to contamination area, pulse energy, beam size, beam divergence,pulse duration, and frequency chirp (arrangement of frequency componentsalong the pulselength). The group velocity dispersion (GVD) parameter ofair, β₂, which is proportional to the rate of change of group velocityof light with wavelength, is positive in the optical and near IRregions. This means that light with longer wavelengths travels fasterthan light with shorter wavelengths. For a pulse with a negativefrequency chirp where shorter wavelengths are at the beginning andlonger wavelengths are at the end, it will compress longitudinally asthe different wavelength components catch up with one another. For apulse in which frequency is a linear function of time, the propagationdistance for maximum longitudinal pulse compression, L_(GVD), isapproximately

$\frac{2{T(0)}}{\beta_{2}\delta\;\omega},$where T(0) is the initial laser pulse duration and δω is the frequencybandwidth of the laser. Therefore, by tailoring the initial laser pulsebandwidth and pulse length, L_(GVD) can be appropriately selected.

As the pulse is shortened, the power increases, as discussed above.There is a threshold power level given by P_(crit)=λ²/2πn₀n₂, abovewhich nonlinear optical effects are induced, where n₀ is the linearindex of refraction, and n₂ is the nonlinear index of refraction. Forlight with λ=800 nm, P_(crit)≈1.7 gigawatts in air. At these and higherpower levels, the refractive index across the transverse profile of thelaser beam is no longer uniform and causes the beam to self-focus. Thenon-uniformity is with respect to the refractive index of air. Therefractive index of air is normally a constant, and varies with densityand temperature. Accordingly, in the small transverse cross-sectionalarea where the laser beam passes through, the air is uniform andconstant. However, for a high enough laser intensity, the refractiveindex is not uniform. The refractive index of air is non-uniformtransversely due to the fact that the high intensity laser is notuniformly transverse. This is a consequence of the nonlinear nature ofthe refractive index.

The characteristic distance for a transverse non-linear self-focusing(NSF) is given by

${L_{NSF} = \frac{z_{R}}{\sqrt{\frac{P(z)}{P_{crit}} - 1}}},{{{where}\mspace{14mu} z_{R}} = \frac{\pi\; n_{0}R^{2}}{\lambda}}$is the Rayleigh range, and R is the beam radius. The transverseself-focusing range, L_(NSF), can then be selected by choosing thecorrect initial beam size, R, and initial beam power, P(0). For aproperly chosen set of parameters the focal distances for the linearlongitudinal compression and the transverse non-linear focusing can bemade to coincide, which results in a significant increase in the laserintensity over a relatively localized region and the onset ofatmospheric breakdown (and plasma formation) from the increased laserintensity at the desired location occurs. This approach controls thelocation of the atmospheric breakdown and the UV radiation generationthat follows.

Longitudinal pulse compression in air occurs for a negatively chirpedlaser pulse (frequency decreases in time). This is because air has anormal (positive) GVD such that high frequency components travel atslower velocities. The back of the negatively chirped laser pulse, whichis composed of the lower frequencies, travels faster than the front andeventually catches up and shortens the pulse length. In a low intensityregime, where nonlinear effects do not significantly affect the temporalshape of the pulse, and assuming Gaussian temporal profile the pulseduration T can be written as a function of the propagation distance z

$\begin{matrix}{{T(z)} = {T_{0}\left( {\left( {1 + {\beta_{0}\frac{z}{Z_{T}}}} \right)^{2} + \left( \frac{z}{Z_{T}} \right)^{2}} \right)}^{1/2}} & (3)\end{matrix}$where T₀ is the initial pulse duration (FWHM), β₀ is the initial pulsechirp, and Z_(T)=T₀ ²/(4 ln 2β₂) is the group velocity dispersionlength.⁷ The GVD parameter, β₂. for atmospheric air is usually estimatedby the updated Edlén equation.⁸ The calculated value of β₂ is ˜21 fs²/mfor typical experimental parameters of air such as a temperature around20 degrees Celsius, a 50% relative humidity, and a laser wavelength of˜800 nm. If the laser pulse is initially negatively chirped (β₀<0), itwill be compressed as it propagates through the atmosphere until itreaches the minimum pulse length of T₀/(1+β₀ ²)^(1/2) at the distance

$z_{s} = {{- \frac{\beta_{0}}{1 + \beta_{0}^{2}}}{Z_{T}.}}$It has been experimentally shown that low energy negatively chirpedlaser pulses can be successfully compressed after propagating in air fora relatively long distance, in good agreement with the linear model. Athigher energy levels, temporal pulse shaping can be significantlyaffected by nonlinear effects such as the self-phase modulation (SPM),but can be potentially controlled through spectral and temporal pulseshaping.

For laser parameters such as a frequency chirp of β₀=˜0.025(corresponding to a bandwidth of 20 nm for a 800 nm Ti:Sapphire laser)and an initial laser pulse length of 4.83 psec, the distance at whichthe laser pulse will be compressed to a 50 fsec pulse is 10 km.

Laser filaments are formed when the IR laser pulse is transverselyfocused resulting in UV radiation generation. Specifically, in highpower ultrashort laser pulse propagation in air there is a dynamicbalance between the nonlinear self-focusing from the air and thedefocusing from the laser-induced plasma, which forms as the laserionizes the air and happens before recombination occurs. Ionization isthe separation of electrons from atoms. This occurs when high intensitylaser radiation interacts with atoms. Ions are formed when electrons arestripped from the atoms. Recombination occurs when the electronsrecombine with the ions to re-form the atoms, once the laser pulse hasbeen passed through and a plasma has formed, and is in the process ofcooling. The energy released during this recombination process shows upas a light and is called the recombination radiation. This radiation hasdifferent wavelength spectra for different atoms and laser intensities.With respect to an embodiment of the present invention, therecombination radiation spectrum is in the ultra-violet range.

This results in a breakup of the laser beam into one or severalfilaments of about 100 μm in diameter that propagates over distances ofseveral meters. Each filament contains a very high intensity core ofabout 10¹³ W/cm², which, in addition to generating broadband white-lightcontinuum ranging from the UV to the mid IR regions, converts part ofthe fundamental frequency to the 3^(rd) harmonic. Currently availablehigh peak power ultrashort lasers usually operate in the near IR regionaround 700-1500 nm (with further potential up to and including 10,000nm.), thus placing the 3^(rd) harmonic in the range of between 233 nmand 500 nm.

FIG. 2 illustrates an example of the UV portion of the self-phasemodulated spectrum of the laser beam containing laser filaments. FIG. 3illustrates the spectrum of the third harmonic of the fundamentalfrequency. The central wavelength of the initial laser spectrum wasapproximately 800 nm. Ultraviolet radiation can be generated in air byan intense laser pulse such as the compressed short infrared laser pulsedescribed in this invention. The generation processes involve nonlinearinteractions between the intense laser radiation with the air. Thenonlinearity originates from the change of the refractive index of airas a function of laser intensities. For a short intense laser pulse withfinite lateral extension, the intensity changes rapidly both in timefrom front to back of pulse and in space (more intense along the laseraxis).

Three processes primarily contribute to the UV production from the IRlaser source. These processes include third harmonic generation,self-phase modulation and recombination of electrons to produce atemporary, or short duration plasma.

Harmonic generation produces overtones of the fundamental as theelectrons bound to the air molecules oscillate in the intense field ofthe laser and execute non-sinusoidal orbits. In air, the strongestemission is the third harmonic. For a short pulse laser with afundamental wavelength of 800 nm, the third harmonic has a wavelength of267 nm which is ultraviolet. This is shown in FIG. 3.

Self-phase modulation produces broad band radiation that extends intothe UV spectral region through the temporal variation of the laser phaseas the laser intensity varies within the pulse. This is shown in FIG. 2.

Finally, ionization and recombination produces UV line emission andbroadband radiation as the photo-ionized atoms reabsorb the electronsand release the energy. These UV radiations constitute the UV sourcesfor exciting fluorescence in the atmospheric contaminants at a distance.

The dominance or suppression of one or more of the three processes canbe conveniently controlled by tailoring the outgoing infrared laserpulse from the short pulse laser. For example, a change in the pulseshape could enhance one process over the other. That could includevarying the final compressed pulse length by adjusting the initialfrequency bandwidth of the laser, altering the frequency chirping in thelaser pulse, and incorporation of additional geometrical focusing opticsin the output beam director, among others. A desired UV spectrum withthe appropriate UV spectral lines can therefore be generated bymanipulating the parameters and configuration of the IR short pulselaser. Most of these parameters can be adjusted in real time. The otherparameters can be preloaded and pre-adjusted for different applicationsand suspected contaminants in the target area.

FIG. 4 shows one of the detection and identification methods that can beapplied for the UV induced fluorescence signature. It is applicable tothe biological surrogate (albumin powder). The decay lifetime of thefluorescence from the albumin powder is a signature that can be pickedup through the use of time resolved spectroscopy. This technique is oneamong several spectroscopic methods of detecting contaminants throughtheir UV induced fluorescence.

Typical energy conversion efficiency to UV region is around 10⁻⁴-10⁻³,which makes laser filaments quite a promising source of locallygenerated UV light. By positioning the laser filaments in the area ofinterest and registering the fluorescent response from this region, thedetection and identification of biological aerosols can be conducted.The major advantage of this method is its capability to position thesource of UV radiation at the desired location through controlling thefilament formation. Since the air absorption and scattering of the laserwavelength in the near infrared region is relatively weak the scanningrange will not be limited as in the case of a direct UV laser source.These filaments can be generated as far as several to over tenkilometers away from the laser. In addition, the relatively short lengthof the filament (on the scale of about ten meters) can allow ranging ofthe contaminated area with good spatial resolution.

Often, the fluorescent spectroscopic signature of the biological agentoverlaps with the broadband white light continuum generated by the shortpulse in air. These spectra can be distinguished by making time-resolvedspectral measurements. As long as the duration of the laser pulse ismuch shorter than the characteristic decay time of the biologicalfluorescence, these spectral signals can be time resolved. Typicalfluorescent decay time is at least a few nanoseconds long, while thetypical laser pulse used in this invention is usually shorter than 1picosecond at the final air breakdown location. By using fast sensingdetectors. the spectral signal from the scattered laser light can beseparated from the fluorescent response. As an example. FIG. 4 shows thetime resolved signal from a photo multiplier tube (PMT) for fluorescent(dashed line) and non fluorescent (solid line) aerosols at thewavelength of interest (340 nm). The difference in these temporalprofiles in the PMT signal allows for the detection of the biologicalagent dispersed in air.

The major advantages of an embodiment of the present invention is theoperation of UV fluorescence lidar using atmospheric propagating laserwavelengths and the control of the location of the UV radiationgeneration through laser pulse chirping and beam collimation. The datacan be accumulated in real time thus allowing the sampling of largecontaminated areas. The lasers and the detectors required for thisapplication are typically commercially available.

Alternatives to an embodiment of the present invention can be made atthe laser and on the methodology. Alternative wavelengths are availablefor the ultrashort femtosecond laser used in this invention depending onthe laser gain materials. At the 700-900 nm region, Titanium Sapphirecan be used. At the 1050 nm region, Neodymium glass and YtterbiumTungstate laser can be used. At the 1550 nm region, Erbium fiber lasersand optical parametric chirped pulsed amplifier (OPCPA) lasers can beused. The 1550 nm wavelength has the added advantage of being“eye-safe”. These lasers all have enough bandwidth to be compressed bythe atmosphere to deliver an ultrashort high power laser pulse at aremote distance to cause filament development and atmospheric breakdown.Another alternative of generating different initial wavelengths is tofrequency double the fundamental wavelengths before the laser pulse istransmitted through the atmosphere. For example, 530 nm femtosecondlaser pulses can be obtained by frequency doubling a 1050 mm ultrashortlaser using the appropriate frequency doubling nonlinear crystal.Similarly, other frequencies such as 400 nm and 775 nm can be obtainedby frequency doubling the 800 mm and 1550 nm lasers. Therefore, limitedtenability can be obtained to tailor to the required wavelength for thespecific chemical or biological compound in question.

The mechanisms for generating the required UV radiation by the filamentsand atmospheric breakdown can have alternatives, in addition to thewhite-light super-continuum and third harmonic generation, such ashigher order harmonic generations appropriate for the associatedfundamental wavelengths of the lasers. For example, the fifth harmonicgeneration is appropriate for the 1050 nm fundamental and the seventhharmonic generation is appropriate for the 1550 nm fundamental forgeneration of UV in the 200 nm region. Efficient high order harmonicgeneration can be achieved by appropriate shaping of the outgoing laserpulse through bandwidth manipulating elements in the laser.

The method of biological agent detection also is not limited to UVinduced fluorescence only. Biological agents are also known to producefluorescence when illuminated with other laser wavelengths such as 530nm which is available as a frequency doubled 1050 nm laser as describedin the previous paragraphs. Other methods of detection that utilizenonlinear interaction of the laser pulse with the biological compoundare also available. For example, two-photon-excitation where two photonsare required to cause fluorescence can be used as proposed in reference[4], where the fundamental wavelength was suggested to be 530 mm tocause fluorescence that could otherwise be generated at the UVwavelength of 265 nm. An embodiment of the present invention will eithergenerate the 530 nm wavelength with the appropriate laser for thetwo-photon excitation, or to produce the needed 265 nm UV light at theremote location using third harmonic with a Titanium Sapphire laser at795 nm.

Another alternative to the laser configuration is the repetition rate ofthe laser. Most standoff detection schemes can benefit from highrepetition rate data collection so that the signal to noise ratio can beimproved through statistical data analysis. However, there are instanceswhere single shot high energy per pulse laser can be useful also.

Although only several exemplary embodiments of the present inventionhave been described in detail above, those skilled in the art willreadily appreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims.

1. A method for detecting atmospheric contamination, said methodcomprising: generating an infrared light beam remotely from anatmospheric contamination region; directing an infrared light beamtoward the atmospheric contamination region; wherein linearlycompressing a longitudinal component of said infrared light beam andincreasing intensity of said infrared light beam: wherein non-linearlycompressing a transverse component of said infrared light beam; whereinconverting said linearly compressed infrared light beam to anultraviolet light source; and wherein producing fluorescence of theatmospheric contamination, when said generated ultraviolet light sourcecontacts the atmospheric contamination at the atmospheric contaminationregion.
 2. A method as claimed in claim 1, comprising detecting saidfluorescence of the atmospheric contaminants via sensors.
 3. A method asclaimed in claim 2, comprising comparing said detected fluorescence withsignatures of known atmospheric contaminants, to determine theatmospheric contaminant.
 4. A method as claimed in claim 3, wherein saidcomparing further includes comparing decay lifetime of the atmosphericcontaminants to the decay lifetime of known contaminants.
 5. A method asclaimed in claim 1, wherein said generating the ultraviolet light beamfurther includes generating a self-phase modulated spectrum of theultraviolet light beam.
 6. A method as claimed in claim 1, wherein saidgenerating the ultraviolet beam further includes generating a spectrumof the third harmonic of a fundamental frequency of the ultravioletlight beam.
 7. A method as claimed in claim 1, wherein said generatingthe ultraviolet light beam further includes forming a short durationplasma via removing electrons from atoms in the atmosphericcontamination region.
 8. A method as claimed in claim 1, wherein saiddetecting fluorescence includes detecting in the visible range.
 9. Amethod as claimed in claim 8, wherein detecting fluorescence in theultra-violet range includes detecting the fluorescence within thewavelength of approximately 330 nm.-700 mm.
 10. A method as claimed inclaim 1, wherein said generating an infrared light beam comprisesgenerating an infrared light beam having a wavelength of approximately700 nm.-1500 nm.
 11. A method as claimed in claim 1, wherein generatingan ultraviolet light beam includes generating an ultraviolet light beamhaving a wavelength of less than approximately 500 nm.